Water Lubrication and/or Jetting is a technique that aids pile installation and is generally associated with silent and vibration free piling methods.
Dense granular materials and all clays above firm are difficult to penetrate by purely jacking the pile. Other techniques are employed to reduce the driving force required, the primarily methods being pre-augering and water lubrication/jetting.
High pressure water is delivered to the pile toe via a 50mm diameter jetting lance which is temporarily secured into the pan of the pile by a disposable clip and a securing weld. A nozzle is fixed to the end of the lance just above the end of the pile. Once the pile is ready to drive, water is delivered by a purpose build pump through the lance to the toe of the pile. In coarse granular soils the soil is disturbed locally at the pile toe by the water and reduces the driving resistance thus aiding pile penetration.
In fine and medium granular soils, the main action of the water is to locally increase the pore water such that the inter particle friction in the soil is virtually eliminated, thus reducing the internal shear strength of the soil immediately under the pile toe. In coarse granular soils, the main action of the water is to oscillate the particles reducing the resistance to pile penetration.
When the steel sheet piles are installed into cohesive soils most of the resistance to pile penetration results from shaft adhesion, (or skin friction), rather then end resistance. The closely spaced platelets in clay soils render the clay effectively impermeable therefore the action of the water cannot be to either oscillate the particles or to significantly affect the pore water pressure.
This impermaeability ensures that the bulk of the water returns to the ground level or the level of the clay strata. The lubricating action of the returning water is clearly very efficient in reducing the amount of adhesion contributing to the pressing resistance whilst at the pile toe the water has a localised softening/cutting effect which reduces the end resistance and eases penetration of the pile.
Water jetting can be used close to buildings but monitoring will be required as ground settlement could occur due to the loss of fines from the soil. Care must be taken when adjacent to poorly constructed basements which could be subject to water penetration. In certain circumstances water jetting can induce heave of the soil due to pressure build up when jetting into granular soil with a impermeable cap above, but we have the expertise to analyse the soils information and advise you of the associated risks.
Further details can be found in our Generic Method Statements under “Services, Other”.